Oxidation behavior of (NbTaZrW)C high-entropy carbide at 800-1000?C

To enrich the database of HECC oxidation and to assess the potential of the hard-and-tough (NbTaZrW)C for high temperature application in oxygen-containing environments, in the current study, the oxidation resistance and behavior of this W-incorporated high-entropy carbide ceramic (HECC) was evaluated and investigated. To achieve this end, isothermal oxidation tests were conducted in a temperature range of 800-1000 degrees C. Natures of the oxidation products were systematically characterized. The results show that, the oxidation of (NbTaZrW)C follows the parabolic rule, suggesting the abeyance of a diffusion-controlled oxidation process at 800-1000 degrees C. Mechanistically, the oxidation of (NbTaZrW)C is dominated by the inward diffusion of O2 with no sign of outward diffusion of cation-site metallic elements. Varying with temperature, a few compositionally complex oxides, including (Nb0.5Ta0.5)2Zr6O17, ((Nb0.5Ta0.5)2O5)15(WO3)2, (Nb0.5Ta0.5)16W18O94, as well as (NbTaZrW)Ox with unknown structure. During oxidation, laminated structures with inter-laminate cracks tend to form due to the combined effects from residual stress and gaseous products. Our final comparison shows that, (NbTaZrW)C exhibits significantly lower oxidation resistance than a few other HECCs, such as (HfTaZrNb)C and (HfZrTaNbTi) C. The low oxidation resistance, as quantified by the extremely high kp values, of (NbTaZrW)C suggest its unsuitability for application in high temperature oxygen-containing environment.